Many electronic appliances such as liquid crystal display monitors need flexible printed circuit (FPC) boards to connect various electronic circuits together. FPC boards are necessary components in many electronic devices having folding portions.
Referring to FIG. 4, a typical FPC board 1 includes a wiring layer 11, an insulating layer 12, and a reinforcing layer 13. The insulating layer 12 covers most of the wiring layer 11 to protect the wiring layer 11 from being exposed to air. The reinforcing layer 13 covers part of the insulating layer 12 to reinforce the mechanical strength of the FPC board 1. The wiring layer 11 includes a plurality of conductive lines 111. The conductive lines 111 are separate from each other, and are generally parallel to each other. Each of the conductive lines 111 includes a line portion 112 and a finger 113. The fingers 113 connect with external circuits or chips. In order to ensure mechanical and electrical contact when the fingers 113 are connected to an external circuit or chip, a width of each finger 113 is greater than a width of the corresponding line portion 112.
Referring to FIG. 5, because the width of the finger 113 is greater than the width of the line portion 112, a junction portion (not labeled) between the finger 113 and the line portion 112 forms sharp right angles. That is, the width of the line portion 112 at one side of the junction portion is much less than the width of the finger 113 at the other side of the junction portion. Thus when the FPC board 1 is bent or the conductive lines 111 are subjected to other stress, the conductive lines 111 are liable to fracture or even break at the junction portions. Therefore the reliability of the FPC board 1 may be unsatisfactory.
Referring to FIG. 6, a strength of the material of the reinforcing layer 13 is greater than a strength of the material of the insulating layer 12 and a strength of the material of the wiring layer 11. In addition, a thickness of the FPC board 1 is nonuniform. For these reasons, tension force at a transverse edge portion of the reinforcing layer 13 at a middle portion of the FPC board 1 is great. Thus the conductive lines 111 are liable to fracture or break at the edge portion when the FPC board 1 is bent. This is another reason why the reliability of the FPC board 1 may be unsatisfactory.
What is needed, therefore, is an FPC board that can overcome the above-described deficiencies.